The LIP-6 monoclonal antibody identifies a novel protein expressed by mature B and myeloid lineage cells and by bone marrow pre-B cells. We have investigated whether LIP-6 can be used to isolate cells that are committed to B cell differentiation but lack the expression of CD45R(B220). The phenotypic identification of cells prior to CD45R(B220) expression would enable the study of mechanisms responsible for hematopoietic cell lineage commitment. A population of bone marrow cells has been identified that is LIP-6+CD45R(B220)- Ly-5(CD45)+++ that has the capacity to differentiate into B, T and myeloid lineages when inoculated into sublethally irradiated mice. This population was further divided phenotypically into L-selectin+ and a L-selectin- populations, comprising 3-4% and 0.5%, respectively, of total bone marrow. In vivo reconstitution studies with these cells has shown that the L-selectin+ cells differentiate only into B lineage cells but have a limited capacity for self renewal. In contrast, the L-selectin- cells were capable of differentiation within the B, T and myeloid lineages and probably contains pluripotent hematopoietic stem cells. Future experiments will determine the frequency of B cell progenitors within the L-selectin+ cells as well as their other phenotypic and functional characteristics. LIP-6, in combination with CD43, L-selectin, and CD45R(B220), has also been used to delineate the terminal stages of B cell differentiation. LIP-6 expression is increased during the immunoblast, plasmablast and immature plasma cell stages, but is lost upon differentiation to the mature plasma cell. In addition, L-selectin- LIP-6+ cells found in Peyer's patches and immunized lymph nodes correspond phenotypically to germinal center B cells. Experiments to verify this are in progress. In addition, attempts to isolate and clone the gene coding for the LIP-6 protein will continue. A B cell line, NFS-1.0, that expresses LIP-6 will be used to construct a cDNA expression library to be screened with the LIP-6 antibody.